Much of the known reserves of oil and gas around the world are found in carbonate formations. To optimize the production of these reserves, petroleum engineers seek to understand the physical properties of these formations, including their porosity and permeability. For many geologic formations, their physical properties are determined primarily as they are deposited, and modified to some extent by pressure and heat. Therefore it is possible to describe and classify such geologic formations in terms of their depositional environments, with some acknowledgement of subsequent changes to the physical properties.
Carbonates, however, present an unusual challenge in that their properties may be greatly modified, and the rock type changed completely, by a process of diagenesis. In particular, the pore system may be very different from that found in the original depositional environment. Carbonates also exhibit larger pores, so-called secondary porosity, where diagenetic processes create larger scale pores or “vugs”. In some carbonates these vugs are connected, and in other carbonates they are not. These additional factors greatly influence the flow of fluids through the geologic formations. If the carbonates have not been modified by diagenesis, the dynamic or flow properties are those of the rocks as deposited and are controlled largely by the pore types related to the initial texture of the rocks. If the carbonates have been modified by diagenetic processes, their dynamic properties are controlled by a combination of primary porosity determined by the pore types, which may differ from that of the originally deposited rocks, and the secondary porosity with its associated pore types.
Incorporation of rock typing in carbonate workflows is dictated by inherent heterogeneity, variation of pore types and significant impact of diagenetic processes. However, existing methods have significant gaps in: (1) incorporating diagenetic processes; (2) accounting for multi-modal pore throat distributions in pore typing; (3) accounting for fractures; (4) integrating dynamic data; (5) accounting for different scales and (6) providing the appropriate geostatistical tools to properly distribute PRTs in the static reservoir model.
Among other things, what is required is a method of rock typing for carbonates that incorporates diagenetic processes, accounts for multi-modal pore throat distribution in pore typing, takes into account the influence of fractures, and integrates dynamic data.